1. Field of the Invention
This invention relates to friction and stiction compensation in spacecraft attitude control. It is particularly useful with spacecraft attitude control systems using reaction wheels that can spin through zero rpm.
2. Prior Art
Momentum or reaction wheels capable of storing angular momentum are known for use as part of a spacecraft attitude control system to provide torque to change or maintain a desired orientation. A particularly difficult problem with this approach is the presence of friction and stiction during the slowing and zero rotation speed which occur during direction reversal of a reaction wheel. These disturbances detract form the ability of the attitude control system to maintain proper attitude. Friction can cause offset pointing of a spacecraft which is reaction wheel controlled. Stiction can occur while the bearings of a reaction wheel come to a halt while the wheel reverses direction. The result is uncertainty in the attitude control response.
Various systems are known for friction and stiction compensation for reaction or momentum wheels used in spacecraft attitude control. One example of a friction compensation system is described in U.S. Pat. No. 5,201,833 (""833) by Goodzeit et al. issued on Jul. 19, 1991. The ""833 patent suggests that wheel bearing velocity reduces the actual torque imparted to the spacecraft in response to a torque command signal. Goodzeit teaches a system of friction compensation provided by applying a torque command signal to a model of an ideal friction-free wheel and calculating the speed the ideal wheel achieves in response to the torque command. An error signal is generated from the difference between the ideal wheel speed and the actual wheel speed. The error signal is summed with the torque command signal to produce the wheel drive signal. With such a system, the actual wheel speed tends toward the ideal speed, thereby causing a torque on the spacecraft which is substantially equal to that commanded.
A system for compensation of stiction in a reaction wheel is described in U.S. Pat. 5,020,745 (""745) by Stetson issued on Jun. 4, 1991. The ""745 patent teaches overcoming the stiction existing in a reaction wheel when the reaction wheel is at a zero velocity state. Stiction is defined as a friction that tends to prevent relative motion between two moveable parts at their null position. The ""745 patent teaches using a dither to overcome stiction. Dither is defined as a force having a controlled amplitude and frequency, applied continuously to a device driven by a servomotor so that the device is constantly in a small amplitude motion and cannot stick at its null position. The use of a dither signal is taught so as to control the energy added to the reaction wheel and reduces attitude errors caused by excessive torque to compensate for stiction during the direction reversal of a reaction wheel.
This invention attempts to minimize or eliminate frictional and stictional perturbations prior to their causing spacecraft attitude perturbations. There is provided an improved means for removing reaction wheel friction or stiction disturbances that detract from the ability of the attitude control system to maintain or shift the spacecraft to a desired attitude. This invention allows these disturbances to be removed before they cause spacecraft attitude changes which are sensed by the attitude control sensors. Preferably this invention comprises a buffer between a spacecraft attitude control subsystem and the torque actuators for applying torque to the reaction wheels. This invention delivers increased attitude pointing precision. This invention, being preferably localized to each wheel, is capable of providing significant additional benefits, including without limitation, safe operating speeds, wheel failure flags, accurate wheel tracking with limited tachometer information and full access to the wheel torques.
In accordance with a preferred embodiment of this invention there is provided a spacecraft attitude control system comprising: at least one reaction wheel responsive to a torque drive signal for torquing the spacecraft; means for generating a torque control signal; means for generating an estimate of friction signal for the at least one reaction wheel; means for detecting if stiction of the at least one reaction wheel is present; means for generating a stiction compensated estimate of friction signal responsive to the detecting means detecting the presence of stiction; and means for compensating for friction or stiction of the at least one reaction wheel by selectively correcting the torque control signal for friction or stiction errors to provide the torque drive signal, wherein the compensating means selectively utilizes the estimate of friction signal to correct the torque control signal when no stiction is present or the stiction compensated estimate of friction signal to correct the torque control signal when stiction is present.
In accordance with the preferred embodiment of this invention there is also provided a spacecraft attitude control process comprising: providing at least one reaction wheel responsive to a torque drive signal for torquing the spacecraft; generating a torque control signal; generating an estimate of friction signal for the at least one reaction wheel; detecting if stiction of the at least one reaction wheel is present; generating a stiction compensated estimate of friction signal responsive to detecting the presence of stiction; and compensating for friction or stiction of the at least one reaction wheel by selectively correcting the torque control signal for friction or stiction errors to provide the torque drive signal, wherein the compensating step selectively utilizes the estimate of friction signal to correct the torque control signal when no stiction is present or the stiction compensated estimate of friction signal to correct the torque control signal when stiction is present.
Accordingly, an object of the present invention is to provide desired wheel speeds for a spacecraft attitude controller whether or not the spacecraft reaction wheels experience friction and/or stiction.
A further object, in accordance with a preferred embodiment of this invention, is to minimize or eliminate frictional and stictional perturbations prior to causing any spacecraft perturbations which would result therefrom.
A still further object, in accordance with another preferred embodiment of this invention, is to provide safe limits for the spacecraft reaction wheel rotational speeds regardless of the torque demands from the spacecraft attitude controller.
A still further object, in accordance with another preferred embodiment of this invention, is to provide estimates of the spacecraft torque demands from the spacecraft attitude controller.
A further object, in accordance with another preferred embodiment of this invention, is to actively control both friction and stiction compensation for spacecraft pointing.
A further object, in accordance with another preferred embodiment of this invention, is to help the spacecraft attitude controller keep control of attitude throughout a reaction wheel stiction event.
A further object, in accordance with another preferred embodiment of this invention, is to provide information concerning the xe2x80x9chealthxe2x80x9d of the reaction wheels to provide warning of their imminent failure,
A further object, in accordance with another preferred embodiment of this invention, is to allow for appropriate attitude control during slow rates of rotation of a reaction wheel when infrequent pulses are provided by a typical pulse tachometer.
A further object, in accordance with another preferred embodiment of this invention, is to permit full reaction wheel torque saturation without unstable or oscillatory side effects.